Protein kinase is a kind of phosphotransferase, and it has the function of transferring γ-phosphate group of ATP to a specific amino acid residue of a substrate, making a protein phosphorylated and exhibit its physiological and biochemical functions. Protein kinase is a kind of important kinase, and it mainly has the following two functions in signal transduction: on one hand, it regulates the activity of proteins by phosphorylation; on the other hand, it amplifies the signals step by step through protein phosphorylation step by step, thereby causing cell response.
Protein kinase abnormal activity not only closely relates to abnormal state of a certain stage in series of intra- or extracellular signal transduction pathways such as tumor proliferation, apoptosis, metastasis, but also a main cause of incurring series of other human diseases associated with inflammation, or proliferation response, for example, rheumatoid arthritis, cardiovascular and neural system diseases, asthma, psoriasis. It is known by now that there are over 400 kinds of human diseases directly or indirectly related to protein kinase, which makes the protein kinase become another kind of important drug target following the G-protein coupled receptor.
Protein kinase as a big family consists of over 500 members, and is generally classified into two types, i.e., protein tyrosine kinases (PTKs) and serine-threonine kinases. According to the position of kinases in a cell, they are classified into receptor kinases and non-receptor kinases, also called intracellular kinases. The receptor kinases generally belong to tyrosine kinases, also called receptor tyrosine kinases (RTKs), such receptor kinases consist of the portion outside cell membrane, transmembrane domain and the portion in the cytoplasm, the catalytic portion of the kinase is located in the cytoplasm. Most of the serine-threonine kinases are located inside the cell, and belong to non-receptor kinases, or called cytosolic kinases.
The typical representatives of the RTKs family are growth factor receptors, which have at least 19 sub-families, the following are the main sub-families:
(a) HER family receptor tyrosine kinases, which include EGFR (epithelial growth factor receptor), HER2, HER3 and HER4. EGFR is a target of synthetic small-molecule drugs such as Tarceva®, Tykerb® and monoclonal antibody Erbitux® for treating non-small cell lung cancer.
(b) Insulin family consists of insulin receptor (IR), insulin-like growth factor I receptor (IGF-1R) and insulin receptor-related receptor (IRR); wherein IGF-1R is a well-known anti-cancer target, since it is too similar to IR, particularly the kinase portion inside the cell, their amino acid sequences are 100% identical, inhibition of the activity of IGF-1R generally also inhibits the activity of IR. It is proved that IR is also an effective anticancer target, since the inhibition of IR may have the risk of increasing blood sugar, and it needs to find a balance between efficacy and safe risk for using IR inhibitor as anticancer agent.
(c) Platelet-derived growth factor receptors (PDGFRs) family, which includes PDGFR-α, PDGFR-β, CSF1R, c-KIT and c-fms; wherein c-KIT is a molecule target of leukemia-treating drugs such as Gleevec® and also for treating gastrointestinal stromal tumors.
(d) Vascular endothelial growth factor receptors (VEGFRs) family, which includes FLT1 (Fms-like tyrosine kinase 1 or VEGFR1), KDR (or VEGFR-2) and FLT4 (or VEGFR3). The members among them are molecular target of Sutent® and Naxavar®.
(e) Fibroblast growth factor receptors (FGFRs) family, which includes FGFR1, FGFR2, FGFR3 and FGFR4 and 7 ligands, FGF1, FGF2, FGF3, FGF4, FGF5, FGF6 and FGF7. The members among them are molecular targets of drugs currently undergoing clinical tests.
(f) MET family, which includes c-Met or called human hepatocyte growth factor receptor (hHGFR), and RON; wherein c-Met plays an important role in the growth and metastasis of the initial tumor, drugs targeting the member of MET family are currently in clinical trials.
(g) RET family, RETs are receptors of a member from GDNF family and have isoforms of RET51, RET43 and RET9, drugs targeting the member of RET family are currently in clinical trials.
(h) Eph family, which is the biggest family of receptor tyrosine kinases, consists of 16 receptors (EPHA1, EPHA2, EPHA3, EPHA4, EPHA5, EPHA6, EPHA7, EPHA8, EPHA9, EPHA10, EPHB1, EPHB2, EPHB3, EPHB4, EPHB5, EPHB6) and 9 ligands (EFNA1, EFNA2, EFNA3, EFNA4, EFNA5, EFNB1, EFNB2, EFNB3). These members play important roles in development of animals, and some of them play roles in tumors.
Non-receptor kinases do not have the portion outside cell membrane and the transmembrane domain, and the whole kinase is in the cytoplasm. By now it is known that there are at least 24 non-receptor kinases, which are divided into 11 subfamilies, i.e. Src, Frk, Btk, CsK, Abl, Zap70, Fes, Fps, Fak, Jak and AcK subfamilies; wherein Src subfamily is the biggest and includes Src, Yes, Fyn, Lyn, Lck, Blk, Hck, Fgr, AUR1, AUR2 and Yrk kinases. For more detailed information, see: Neet, K.; Hunter, T. Genes to Cells 1996, 1, 147-169 and the documents cited therein. Although several non-receptor kinases belong to tyrosine kinases, most of the non-receptor kinases belong to serine-threonine kinases; wherein several members of them are molecular targets of leukemia-treating drugs such as Gleevec® and Sprycel®.
As stated above, receptor kinases and non-receptor kinases as antitumor targets have been fully proved in clinical and practical applications, and several antitumor drugs are approved to sale on the markets to be used for patients. in addition to treatment of tumors, inhibiting abnormal activity of receptor kinases and non-receptor kinases may also be used for treating the following diseases, which include, but not limited to: psoriasis, hepatic cirrhosis, diabetes, angiogenesis-related diseases, restenosis-related diseases, eye diseases, age-related macular degeneration, rheumatoid arthritis and other inflammatory diseases, immune system diseases such as autoimmune disease, cardiovascular diseases such as atherosclerosis, or kidney disease etc. Thus it is needed to go on developing desired inhibitors of these kinases.